A series of experiments are proposed for investigating the manner in which prelinguistic infants process speech and speech-like acoustic stimuli. Discrimination and identification data will obtained from five-to-ten-month-old infants for synthetic and natural speech sounds as well as for nonspeech stimuli generated to match the complex temporal and frequency relations present in speech. Measurement of discrimination and identification will be performed using a conditioned head-turning procedure in which the infant makes an anticipatory orienting movement to obtain a visual reinforcer. Major variables of interest are the perception of the acoustic cues for voicing and place of articulation in stop consonants, the perception of the acoustic cues for vowels, and the influence of the surrounding context and the phonetic environment on the perception of stop consonant and vowel categories. In addition, several experiments are aimed at determining whether certain phonetic features underlie the perception of stop consonants by infants and whether infants show perceptual constancy for stop consonants and vowels containing radically different acoustic cues. Results from these studies will provide important evidence for evaluating several recent claims that speech sounds are processed in a specialized perceptual mode and that the perceptual system of infants is innately attuned to analyze speech sounds in a language-specific manner.